and <math>{{\beta }_{n}}</math> is the eigenvalue of the corresponding homogeneous problem. The Nusselt number based on the total heat flux at the external wall is

and <math>{{\beta }_{n}}</math> is the eigenvalue of the corresponding homogeneous problem. The Nusselt number based on the total heat flux at the external wall is

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where <math>{{\theta }_{w}}</math> and <math>{{\theta }_{m}}</math> are dimensionless wall and mean temperatures, respectively.

where <math>{{\theta }_{w}}</math> and <math>{{\theta }_{m}}</math> are dimensionless wall and mean temperatures, respectively.

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The Nusselt number based on the convective heat transfer coefficient is

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The Nusselt number based on the convective heat transfer coefficient is

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Revision as of 04:56, 23 July 2010

There are many transport phenomena problems in which heat and mass transfer simultaneously occur. In some cases, such as sublimation and vapor deposition, they are coupled. These problems are usually treated as a single phase. However, coupled heat and mass transfer should both be considered even though they are modeled as being single phase. In this article, coupled forced internal convection in a circular tube will be presented for both adiabatic and constant wall heat flux.

Sublimation inside an Adiabatic Tube

Sublimation in an adiabatic tube.

In addition to the external sublimation, internal sublimation is also very important. Sublimation inside an adiabatic and externally heated tube will be analyzed. The physical model of the problem under consideration is shown in figure to the right [1]. The inner surface of a circular tube with radius ro is coated with a layer of sublimable material which will sublime when gas flows through the tube. The fully-developed gas enters the tube with a uniform inlet mass fraction of the sublimable substance, ω0, and a uniform inlet temperature, T0. Since the outer wall surface is adiabatic, the latent heat of sublimation is supplied by the gas flow inside the tube; this in turn causes the change in gas temperature inside the tube. It is assumed that the flow inside the tube is incompressible laminar flow with constant properties. In order to solve the problem analytically, the following assumptions are made:
1. The entrance mass fraction, ω0, is assumed to be equal to the saturation mass fraction at the entry temperature, T0.
2. The saturation mass fraction can be expressed as a linear function of the corresponding temperature.
3. The mass transfer rate is small enough that the transverse velocity components can be neglected.

Equation (7) implies that the latent heat of sublimation is supplied as the gas flows inside the tube. Another boundary condition at the tube wall is obtained by setting the mass fraction at the wall as the saturation mass fraction at the wall temperature [2]. According to the second assumption, the mass fraction and temperature at the inner wall have the following relationship:

The heat and mass transfer eqs. (10) and (11) are independent, but their boundary conditions are coupled by eqs. (14) and (15). The solution of eqs. (10) and (11) can be obtained via separation of variables. It is assumed that the solution of θ can be expressed as a product of the function of η and a function of ξ, i.e.,

To solve eqs. (19) and (24) using the Runge-Kutta method it is necessary to specify two boundary conditions for each. However, there is only one boundary condition for each: eqs. (21) and (25), respectively. Since both eqs. (19) and (24) are homogeneous, one can assume that the other boundary conditions are
Θ(0) = Φ(0) = 1 and the solve eqs. (19) and (24) numerically. It is necessary to point out that the eigenvalue, β, is still unknown at this point and must be obtained by eq. (27). There will be a series of β which satisfy eq. (27), and for each value of βn there is one set of corresponding Θn and Φn functions .

If we use any one of the eigenvalues, βn, and corresponding eigenfunctions, Θn and Φn, in eqs. (22) and (23), the solutions of eq. (10) and (11) become

where Tm and ωm are mean temperature and mean mass fraction in the tube.

Nusselt and Sherwood numbers for sublimation inside an adiabatic tube.

The figure to right shows heat and mass transfer performance during sublimation inside an adiabatic tube. For all cases, both Nusselt and Sherwood numbers become constant when ξ is greater than a certain number, thus indicating that heat and mass transfer in the tube have become fully developed. The length of the entrance flow increases with an increasing Lewis number. While the fully developed Nusselt number increases with an increasing Lewis number, the Sherwood number decreases with an increasing Lewis number, because a larger Lewis number indicates larger thermal diffusivity or low mass diffusivity. The effect of (ahsv / cp) on the Nusselt and Sherwood numbers is relatively insignificant: both the Nusselt and Sherwood numbers increase with increasing (ahsv / cp) for Le < 1, but increasing(ahsv / cp) for Le > 1 results in decreasing Nusselt and Sherwood numbers.

Sublimation inside a Tube Subjected to External Heating

Figure 3: Sublimation in a tube heated by a uniform heat flux.

When the inner wall of a tube with a sublimable-material-coated outer wall is heated by a uniform heat flux, q''(see figure to the right), the latent heat will be supplied by part of the heat flux at the wall. The remaining part of the heat flux will be used to heat the gas flowing through the tube. The problem can be described by eqs. (1) – (8), except that the boundary condition at the inner wall of the tube is replaced by

where in eq. (43).
The sublimation problem under consideration is not homogeneous, because eq. (45) is a nonhomogeneous boundary condition. The solution of the problem is consistent with its particular (fully developed) solution as well as the solution of the corresponding homogeneous problem [3]:

While the fully developed solutions of temperature and mass fraction, θ1(ξ,η) and , respectively, must satisfy eqs. (40) – (41) and (44) – (46), the corresponding homogeneous solutions of the temperature and mass fraction, θ2(ξ,η) and , must satisfy eqs. (40), (41), (44), and (46), as well as the following conditions:

The variations of the local Nusselt number based on total heat flux along the dimensionless location ξ are shown in the figure to the right. It is evident from part (a) of the figure that Nu' increases significantly with increasing (ahsv / cp). The Lewis number has very little effect on Nux when (ahsv / cp)= 0.1, but its effects become obvious in the region near the entrance when (ahsv / cp) = 1.0 and gradually diminishes in the region near the exit. has almost no influence on Nu in almost the entire region when (ahsv / cp) = 1.0, as seen in part (b) of the figure. When (ahsv / cp)= 0.1, Nux increases slightly when ξ is small.

Nusselt number based on convective heat flux and Sherwood number.

The variation of the local Nusselt number based on convective heat flux, Nu*, is shown in part (a) of the figure to the right. Only a single curve is obtained, which implies that Nu* remains unchanged when the mass transfer parameters are varied. The value of Nu* is exactly the same as for the process without sublimation. Part (b) of the figure
shows the Sherwood number for various parameters. It is evident that (ahsv / cp) and have no effect on Shx, and Le has an insignificant effect on Shx in the entry region.